In power-assisted steering units with open center or OC steering valves, if the steering valve is in neutral position, the pump pumps a pressure fluid at low pressure through the open valve into a reservoir. The steering valve serves to supply hydraulic pressure to a control member comprising, in a manner known per se, an input shaft, an output shaft, a torsion rod connected by its one end to the input shaft and by its other end to the output shaft, and a valve sleeve surrounding the input shaft. The input shaft is connected with a steering rod while the output shaft engages with a rack by means of a pinion. If the input shaft is twisted in relation to the valve sleeve, hydraulic pressure is provided to support the steering motion in the corresponding direction. It is known to use a separate electric motor to drive the pump that delivers the pressure fluid such that on the one hand steering is available even if the drive motor of the motor vehicle is shut off and on the other hand the power consumption of the electric motor can be electronically controlled to permit power output to the pump as needed. Such a steering system permits a reduction of up to 75% in the proportional gasoline consumption of a power-assisted steering unit. The capacity of the electric motor pump arrangement is decisively determined by the power input, which is limited by the automobile industry. It is approx. 60 to 80 ampere for an electrical system voltage of typically 13.5 volt. This limit does not significantly affect the power-assisted steering of smaller vehicles with a front axle load of less than 600 kg. But with heavier vehicle classes (900 to 1200 kg) or high steering reduction ratios it clearly limits the performance compared to conventional power-assisted steering units. This is expressed by the so-called "catch-up effect" which, on rapid turning of the steering wheel, for example while the vehicle is stationary, causes the driver to momentarily notice a sluggish steering response because the steering volume flow decreases above a critical point. With a demand-based design of the motor pump arrangement, the maximum performance of the steering system cannot be increased if power consumption is subject to a given maximum limit.
DE 44 09 928 A1 discloses a hydraulic actuator for axle steering of a motor vehicle distinguished by an increased dynamic in the steering operation with reduced energy consumption. This is achieved in that an OC steering valve is pressurized with a pressure fluid from a pressure fluid supply unit comprising an electric motor driven pump and a hydraulic accumulator that is hydraulically separated from the steering valve by a stop valve designed as a seat valve. A pressure sensor monitors the pressure of the hydraulic accumulator and the corresponding signals are used to control the electric motor. A flow control valve is used to divert a steering fluid flow and to deliver it to the steering valve via a stop valve bypassing the steering control. The pressure increase produced by actuating the steering valve causes the stop valve to open so that an additional volume flow passes from the hydraulic accumulator to the steering valve to support the steering motion.
The disadvantage of the prior art hydraulic actuator is that activation of the steering valve causes oil to be continuously withdrawn from the hydraulic accumulator, which is under high pressure even if the steering pressures are small. This has a negative effect on the energy balance since the throttling losses cause unnecessary heating of the oil. Consequently, the hydraulic accumulator must be frequently recharged, which increases the average power requirement of the electric motor. This, in turn, negatively affects the proportional gasoline consumption of the steering system compared to the above described 75%.
The object of the invention is to further develop a method of the initially described type and, within the current framework prescribed by the automobile industry, to provide a steering volume flow based on need which exceeds the maximum pump output without increasing average power consumption.